This invention relates to a continuous electric tunnel furnace of rectangular tunnel cross section. The electric heating elements of the furnace are generally arranged under the tunnel roof. The furnace further comprises an arrangement which blows a nitrogen jet from the tunnel outlet in a direction opposite to the direction of advance of the articles to be fired, particularly ceramic components for electronic equipment.
In continuous tunnel furnaces of the above-outlined type, the green ceramic articles are positioned on slide plates which are advanced continuously through the tunnel furnace and thus the articles are fired in the tunnel during their continuous advance therethrough. Instead of sliding plates, carriages, sleds or the like may be used as carriers for feeding the articles through the tunnel.
In tunnel furnaces of the above-outlined type it is of great importance to ensure, in all furnace zones along the length of the tunnel, a temperature distribution which at every location along the tunnel length is uniform throughout the entire tunnel cross section. The furnace temperature at the furnace inlet is practically identical to the environmental temperature and amounts to, for example, 20.degree. C. In the subsequent heating zone (heat-up zone), the furnace temperature slowly and continuously increases to the sintering temperature of, for example, 1300.degree.-1500.degree. C. This sintering temperature is maintained in the entire sintering zone (high-temperature zone) which is downstream of the heat-up zone (as viewed in the direction of article advance through the furnace). Downstream of the sintering zone the cooling zone begins in which the temperature is continuously decreased to the furnace outlet. At the furnace outlet the furnace temperature is, for example, approximately 100.degree. C.
Similarly to the above-noted desired uniform temperature distribution over any given tunnel cross section, it is important that a uniform furnace atmosphere prevail throughout the entire tunnel cross section at a given location along the tunnel length. The furnace atmosphere is formed of an oxygen-nitrogen mixture. The oxygen-nitrogen ratio at the furnace inlet corresponds to that of the environmental atmosphere. Progressing through the furnace, the oxygen proportion is decreased by introducing nitrogen or other protective gas (since the furnace is heated electrically, no oxygen consumption by the heater occurs). In the sintering zone the oxygen content in the furnace atmosphere is lowered to approximately 2 to 10%. From the beginning of the cooling zone the oxygen content must be less than 0.01%, that is, in the cooling zone the oxygen has to be practically entirely eliminated from the furnace atmosphere. This is effected by blowing nitrogen into the tunnel outlet in a direction which opposes the direction of advance of the articles through the furnace.
The introduction of a nitrogen jet into the cooling zone has, in conventional tunnel furnaces, the following effects on the articles passing through the sintering zone:
The articles positioned on sliding plates or the like extend close to the side walls of the tunnel but have, as a rule, a greater distance from the tunnel roof; one reason being that between the tunnel roof and the articles there are positioned components of the electric heater. The free height depends from the structural height of the articles. At the height level of the articles, the nitrogen stream has, as a rule, a smaller flow velocity in the vicinity of the lateral tunnel walls than in the central area of the tunnel cross section. The reason for this phenomenon is the deceleration effect of the friction between the side walls of the tunnel and the nitrogen flow. The leading (front) flow pattern of the nitrogen therefore has, at the height level of the articles, as viewed over the width of the tunnel, the shape of a parabola bulging substantially in the direction opposite to that of article feed. This means that in the sintering zone in the central cross-sectional area portion of the tunnel the oxygen content of the furnace atmosphere is, because of the above-described non-uniform cross-sectional distribution of the nitrogen jet, reduced to a greater extent than in the two lateral cross-sectional area portions flanking the central area portion. Thus, in the sintering zone the oxygen-nitrogen ratio is not uniform over the entire furnace cross section at the individual points along the path of article feed. This circumstance adversely affects the uniformity of the quality of the fired articles.